In the 1940s, Fago's Funny Features created many cartoon characters, including Loony Leo. These cartoon films were distributed to movie theaters and shown as filler pieces between feature films. The Loony Leo cartoons starred Leo, his rascally nephews, and Leo's chaste romance with lioness Lola.
Loony Leo's real life began in 1946, an unexpected side effect emerging from a theater poster during the discharge of Professor Borzoi's Belief Ray invention. A device which transformed movie characters from imaginary screen-life into the reality of the living world. Borzoi used it to terrorize film-goers. The Gentleman was barely holding his own against one such giant gorilla monster when Leo, now fully corporeal, intervened, taking the Belief Ray away from Professor Borzoi and smashing it.
This act caused the film monster to recede from existence. It also, almost, sent Loony Leo with it. Acting on instinct, the Gentleman called for the crowd to intervene by firmly, consciously, believing in Leo. The focused effort worked, permanently. Leo now doesn't age, can't seem to die, or be destroyed.
Loony Leo crossed the Gentleman's path several times after that, always to the Gentleman's benefit.
Fago's Funny Features then attempted to take ownership of Loony Leo on the grounds that they had created him. This case was ultimately settled out of court, but rulings issued before the settlement occurred have become important precedents, cited in later court cases involving artificial beings.
When Fago's Funny Features offered Leo a chance at a starring in full length movies, he went to Hollywood, making live-action Loony Leo films like Leo's Trip to the Moon. Initially very successful, this gimmick eventually grew old. By 1954, the box office of his final film, Panama Leo was so disappointing that Leo was dropped from the Fago stable.
Leo then became the host and emcee of Loony Leo's Fun-Time Farm, a children's TV show that was immediately cancelled in 1957, following the scandal surronding the death of a very young Corliss McBride. While Leo was not indicted, being even remotely connected to a young hooker who died from a drug overdose was enough to force him out of television and pretty much ended his public career.
Leaving Hollywood, Leo wandered across America for several years until he encountered an alien named Zzardo. That chance meeting led to Leo's involvement in The Myth-Master Incident.
Leo then faded from view for over twenty years, until the opening of Loony Leo's Restaurant in Astro City. He continues to be the host of the restaurant to this day, as well as making occasional appearances in television ads. Time can indeed be a forgiving soul.